1. Field of the Invention
The present invention relates to an apparatus used in nuclear magnetic resonance (NMR) imaging, and more particularly, to a new and improved imager for use in a toroid cavity detector so that in situ magnetic resonance analysis can be accomplished of samples such as battery components, membranes and thin film materials.
2. Background of the Invention
Nuclear magnetic resonance (NMR) techniques have been used as a nondestructive assay (NDA) of various materials. One NMR technique that has been used involves a toroid cavity probe. This technique enables the determination of both the presence and quantity of individual elements as well as full chemical-shift resolution (i.e., chemical speciation). For example, the toroid cavity probe has been used to determine the presence of hydrogen in materials and as to whether hydrogen is bound with oxygen (water) or another element such as a salt. At the same time, protons in other forms, such as hydrides, hydrates and acids, also are detectable and quantifiable. The ability to make such determinations and the sensitivity of toroid-based NMR imaging makes this type of technique ideal for analyzing certain types of material particularly when the materials are in disposed in various types of packaging or containers.
In one type of toroid cavity detector, a sample to be analyzed is placed in the toroid cavity and the toroid cavity is placed in an externally applied static main homogeneous magnetic field (B0). The presence of the B0 field causes the magnetic moments of a targeted class of nuclei in the sample to precess about the field""s axis at a rate which is dependent on the magnetic field strength. Another magnetic field (B1) is produced that is perpendicular to the B0 field and is alternately energized and de-energized within the cavity. In the case where the B1 field is produced by a radio frequency (RF) transmitter pulse applied to a wire or rod (working electrode) extending along the central axis of the cavity, the net sample magnetization is caused to rotate about the B1 field axis. Following the RF pulse, the response of the sample to the applied field is detected and in most applications, received signals from the energized nuclei serve as input signals for spectroscopic analysis of the sample.
One of the unique features of the toroid cavity detector compared to some other types of detectors is the full containment of the RF magnetic field flux that is generated when the working electrode is pulsed with an RF current. As a result, the RF magnetic field flux that emanates from a sample contained within the toroid cavity and subjected to an excitation pulse of electromagnetic radiation is completely captured (i.e., detected). This feature makes the toroid cavity detector two to four times more sensitive to weak nuclear resonance signals than conventional coil resonators and allows a quantitative measurement of the total number of NMR-active nuclear spins in the sample, not possible with other types of NMR detectors.
For samples placed in a magnetic field of sufficiently high homogeneity, it is possible to record different chemical species containing a common nucleus. Not only can a proton be distinguished from a uranium nucleus, but also protons in water molecules can be distinguished from protons in hydrogen gas molecules. In fact, protons in water molecules that are in different physical environments can be distinguished. The resolving capability of chemical shift makes the toroid cavity detector a useful device for analyzing and monitoring various types of radioactive materials from spent nuclear fuel to plutonium ash.
The toroid cavity detector also differs from other, more conventional electromagnetic detectors in that it produces a gradient in the generated RF magnetic field. This B1 field gradient has a mathematically well-defined spatial distribution. This magnetic field gradient feature has two significant consequences. First, the sensitivity of the toroid cavity is radially distributed, with the greatest sensitivity near the central axis of the device or a principal detector element located in the toroid cavity along the central axis. This enables measurements on samples of limited quantity. Second, concentric annular regions of a sample contained in the toroid cavity exchange energy with the resonator circuit at different rates. Thus, analysis of energy transfer rates in a toroid cavity can yield a radial spatial mapping of the different nuclear constituents in a cylindrical sample container. The toroid cavity also has been proven to function with asymmetric conductors.
As is disclosed in U.S. Pat. Nos. 5,574,370 and 6,046,592, the toroid cavity of a toroid cavity detector can be a cylindrically shaped, hollow housing which is closed at both opposite ends and made for example of copper. A central conductor or working electrode (for example, an inner wire or rod in a Teflon outer jacket) extends through the base of the cylinder and is positioned coincident with the major, central axis of the cylinder. In the case of the toroid cavity disclosed in the xe2x80x2370 patent, a fluid or gas sample to be analyzed by the toroidal cavity detector is simply introduced into the toroid cavity whereas in the case of the toroid cavity detector disclosed in the xe2x80x2592 patent, the fluid or gas sample to be analyzed is introduced into an electrochemical cell compartment which is disposed within the toroid cavity. This electrochemical cell compartment consists of a cylindrically shaped glass container along the central axis of which extends the working electrode of the toroid cavity detector and into which a counter electrode is disposed. The counter electrode consists of a cylindrical solenoid coil that is positioned in the electrochemical cell compartment (either at the base of the compartment or expanded along the walls of the compartment) so that it is symmetrical about the centrally located working electrode. In operation, voltages are applied across the working electrode and the counter electrode by means of an external potentiostat resulting in the chemical composition of the sample being changed adjacent to the working electrode within the electrochemical cell compartment. A NMR spectrometer records the response of the sample to the applied magnetic field generated by the toroid cavity detector when a RF frequency signal is applied to the working electrode. The use of such an electrochemical cell compartment within the toroid cavity detector allows for the measurement of macroscopic transport properties, local dynamics, and chemistry of ions as a function of distance from the electrode.
In the toroid cavity detector of the type disclosed in the ""370 and ""592 patents, the B1 field is completely confined within the cavity formed in the housing of the toroid cavity detector and is generated when an RF signal is transmitted along the central conductor. The B1 field is strongest near the central conductor and drops off as the inverse of the distance toward the outer wall of the toroid cavity. Both sensitivity and distance resolution increase with the gradient in the B1 field and consequently, the toroid cavity detector provides a means to image simultaneously both the chemical shift of the targeted nuclei as well as their radial distance from the center and is well suited for NMR microscopy of films that uniformly surround the central conductor. In this regard, the amount of energy that is absorbed or transmitted by a sample within the toroid cavity detector varies with the location of the sample within the toroidal cavity such that multiple distances within the NMR active sample can be resolved by varying the transmitter pulse length. Because a homogeneous B0 field is used, the chemical shift information is not destroyed by the imaging process as happens in conventional magnetic resonance imaging (MRI) where transient inhomogeneous B0 fields are required. In addition, such toroid cavity detectors appear to be useful for investigations of solids and polymers where broad lines usually limit the spatial resolution to 50-100 xcexcm. High-pressure and high-temperature capabilities of toroid cavities enable measurements of penetration rates of gaseous molecules in polymer or ceramic films in situ. In most instances, electrochemical processes can be monitored when the central conductor is used as a working electrode.
The toroid cavity detectors of the type disclosed in the ""370 and the ""592 patents have the advantage of providing a rugged reaction chamber that is readily machined from a variety of alloys, and the alternating magnetic field is highly confined within the cavity detector. Confining the alternating magnetic field minimizes sensitivity losses that occur through magnetic coupling with a high-pressure housing. Most importantly, the toroid cavity detector produces a well-defined magnetic field gradient, which, as noted above, varies with the inverse of the radial displacement from the center of the cavity. The resultant NMR intensity (I/I1) is predicted to depend on the transmitter pulse length, t, according to the following equation:       I    /          I      0        =      2    ⁢    π    ⁢          xe2x80x83        ⁢    h    ⁢                  ∫                  r          1                          r          2                    ⁢                        sin          ⁡                      (                                          -                γ                            ⁢                              xe2x80x83                            ⁢              A              ⁢                              xe2x80x83                            ⁢                              t                /                r                                      )                          ⁢                  ⅆ          r                    
where xcex3 is the gyromagnetic ratio, h is the height of the toroid, r is the radial distance from the center of the cavity, r1 is the radius of the central conductor, r2 is the inside radius of the cavity, and A is the proportionality constant defining the magnetic field as follows:
B1=A/r 
The 1/r relation for the B1 field suggests that both the NMR sensitivity and the distance resolution should increase for materials that are close to the central conductor. Thus, the toroid cavity NMR resonator or detector is particularly powerful in the characterization of surface layers applied to the central conductor.
The use of a toroid cavity detector enables complete NMR spectral information to be retained during signal processing. Additionally, the strong gradient that is intrinsic to the torus enables the toroid cavity detector to provide a theoretical spatial resolution that is better than is possible with conventional MRI. Also, spatial resolution with the toroid cavity imager is less dependent on the line widths of the NMR signals used in the measurements because chemical shift information is not used to determine the distance as it is in conventional MRI.
The toroid cavity detectors of the type disclosed in the ""370 and ""592 patents provide the above discussed and other significant advantages. However, the toroid cavity detectors disclosed in those patents are limited to analyzing objects that are of a cylindrical configuration or can be deposited on, can be affixed to or otherwise can surround the cylindrically shaped working electrode that extends along the elongated central axis of the toroid cavity. However, it would be advantageous to utilize a toroid cavity detector for in situ magnetic resonance analysis of samples that cannot be necessarily deposited on or affixed to a cylindrical detector element of the toroid cavity detector. One such sample is a disk electrode coin cell battery.
Coin cell batteries have become more in demand due to the increase in portable compact electronic equipment. In particular, lithium-ion rechargeable batteries have been the focus of intense research. These batteries are concentrated cells powered by the transfer of lithium ions between two electrodes that are composed of lithium-containing compounds at different concentration levels and therefore having different lithium activities. The use of carbonaceous materials as anode electrodes offers a number of advantages such as the prevention of deterioration of the electrode caused by branchlike growth of metallic lithium upon charging; the increase of the whole-cell cycle lifexe2x80x94making longer-lasting batteries feasible; and the improvement of the reliability, minimizing the risk of explosion from volume expansion associated with using and recharging the batteries. The lower specific capacity of carbon as opposed to lithium metal is compensated for by the use of high voltage cathode materials.
A lithium-ion coin cell battery is encased in a stainless steel housing and includes a series of stacked discs. For example, the coin cell battery could include a lithium negative electrode, a microporous membrane as a separator (an insulator between opposed electrodes of the battery) and a carbon positive electrode. In many instances, the carbon is a thin film that is deposited on the inner surface of the positive electrode. In order to analyze the carbon electrode in a toroid cavity detector, attempts were made to reproduce the carbon used on the electrode and apply it as a film onto the working electrode of the toroid cavity detector. While such analyses provide some information as to the carbon electrode material, the film that was deposited on the working electrode did not necessarily duplicate the film that was actually used in the coin cell batteries and certainly the film was not being analyzed as the battery was being used (for example, at the different levels to which the battery was charged or discharged).
In view of the fact that the elements of a coin cell battery need to be oriented within a toroid cavity detector so that the planes of the elements (disks) are parallel to the axis of the working electrode of the toroid cavity detector, it is not feasible to use the toroid cavity detectors such as those disclosed in the ""370 and ""592 patents to obtain in situ NMR analysis of such coin cell batteries. This is in part due to the fact that the principal detector element of such toroid cavity detectors (i.e., the working electrode) is generally cylindrical in shape and the coin cell battery disk-shaped components cannot be conformed to that shape or affixed relative thereto in an appropriate orientation. Consequently, it would be advantageous to devise a modified principal detector element to be able to analyze such non-cylindrical elements within a toroid cavity detector.
Accordingly, it is an object of the present invention to provide a new and improved toroid cavity detector having a principal detector element that is shaped such that non-cylindrical samples can be analyzed.
It is another object of the present invention to provide a new and improved toroid cavity detector having a flat metal principal detector element to which components of a coin cell battery can be held in electrical contact for in situ magnetic resonance analysis.
It is still another object of the present invention to provide a new and improved toroid cavity detector having a flat metal principal detector element to which samples can be held for in situ magnetic resonance analysis and simultaneously view or probe visible light.
It is yet another object of the present invention to provide a new and improved toroid cavity detector having a sample relative to a principal detector element.
It is still a further object of the present invention to provide a new and improved toroid cavity detector having a flat metal principal detector element to which membranes and thin film materials can be held for in situ magnetic resonance analysis including where the materials can be subjected to different kinds of light to analyze absorption or fluorescence.
In accordance with these and many other objects of the present invention, a toroid cavity detector or resonator includes an outer cylindrical housing having a top and base portion that forms a cylindrical cavity. A first electrical conductor extends from a potentiostat through the top of the housing, along the central longitudinal axis of the housing to a principal detector element (for example, an alloy disk or a carbon coated metallic disk). A second electrical conductor extends from the principal detector element along the central longitudinal axis of the housing and through the base of the housing to an NMR spectrometer. The principal detector element is a flat metal conductor in a regular or irregular shape. In order to analyze a sample held adjacent to or in contact with the principal detector element within the housing, the housing is placed in an externally applied static main homogeneous magnetic field (B0) produced by a NMR magnetic device, the magnetic field (B0) being oriented perpendicular to the surface normal of the principal detector element. The presence of the B0 field causes the magnetic moments of a targeted class of nuclei in the sample to precess about the field""s axis at a rate which is dependent on the magnetic field strength. An RF excitation signal pulse is supplied through the first and second electrical conductors and the principal detector element from the NMR spectrometer such that an alternately energized and de-energized magnetic field (B1) is produced in the sample and through the toroid cavity. This B1 field is oriented perpendicular to the B0 field and causes transitions between adjacent (allowed) or non-adjacent (unallowed) magnetic energy levels of electronic or nuclear origin. The resulting fluctuating magnetic and/or electric fields emanating from the sample induce alternating signal currents in the principal detector element which are detected during, after or interspersed with the application of the alternating excitation signal pulse. The detected signals are recorded by an analog-to-digital converter with a high sampling rate, stored in a computer and analyzed for quantitative speciation (elemental/chemical composition) of different nuclides. Quantitative speciation can be obtained for the bulk sample or as function of the position of the sample away from the surface of the principal detector element in the direction normal to the surface of the principal detector element.
One embodiment of the present invention includes a coin cell battery imager which is adapted to be mounted within the toroid cavity relative to the principal detector element. In this embodiment, the principal detector element is a circular, generally flat metal conductor that is disposed in a circular recess in a non-conductive cylindrical holder. The holder additionally includes a groove which surrounds the recess and which is adapted to either form or receive an O-ring and threaded holes extending through the holder radially outward of the groove. A non-conductive cell cover or end cap is adapted to be secured to the holder by non-conductive screws such that it functions as a piston assembly compressing circularly shaped components (disks) of a coin cell battery (including a carbon electrode, a permeable electrical separator, electroactive metal or counter electrode (lithium) and a metallic solid or mesh current collector) and a liquid electrolyte between the holder and the cap. With the cap secured to the holder, the carbon electrode is forced against the principal detector element (which serves as the direct current collector) and the coin cell battery components are hermetically sealed with the electrolyte due to a flange on the inside surface of the cap compressing the O-ring in the groove in the holder. The mesh current collector is coupled to the potentiostat by a wire that protrudes through the end cap into engagement with the mesh current collector. The battery formed by the coin cell imager is operated by a direct current (DC) potential being supplied from the potentiostat. The circuit for supplying the DC potential includes the first and second electrical conductors, the principal detector element, the coin cell battery components disposed within the coin cell imager and the wire connected between the mesh current collector and the potentiostat. As a result, the battery components can be subjected to charging and discharging by the potentiostat just as they would be when a normal coin cell battery is being operated so that in situ NMR analysis of the battery components can be accomplished with the analysis being made of the components at different dynamic conditions of the battery components as the battery is being charged or discharged.
In another embodiment of the present invention, the circular disk components of a coin cell battery are encircled by an O-ring made of chemically compatible material and the O-ring and coin cell battery components (including a carbon electrode, a permeable electrical separator, electroactive metal or counter electrode (lithium) and a metallic solid current collector) and an electrolyte are disposed between the principal detector element (in this case, a generally flat circular metal conductor) and another circular non-permeable solid electrically conductive contact member. A non-conductive piston or compression assembly is used to compress the O-ring and the battery components between the principal detector element (a circular, generally flat metal conductor) and the other contact member, also known as the counter electrode. As a result, the battery components and the electrolyte are hermetically sealed within the compressed O-ring. A contact wire is extended through the piston assembly so that the battery can be operated by a potentiostat coupled to the counter electrode and the contact wire. As a result, the battery components can be subjected to charging and discharging by the potentiostat just as they would be when a normal coin cell battery is being operated so that in situ NMR analysis of the battery components can be accomplished with the analysis being made of the components at different dynamic conditions of the battery components as it is being charged or discharged.
In still another embodiment of the present invention, the principal detector element is a generally oval-shaped, flat metal conductor that is secured to electrical conductor rods and disposed within the toroid cavity. In this embodiment, a sample (gas, liquid, or solid) is encircled by an O-ring. The sample and O-ring are positioned between one flat side of the principal detector element and a glass plate. Another glass plate is positioned against the opposite flat side of the principal detector element. The glass plates are compressed towards each other by the tightening of nonconductive screws that extend through aligned holes in the glass plates into nonconductive nuts. As a result, the sample and O-ring are compressed against one flat side of the principal detector element and the sample is hermetically sealed between the glass plate and the principal detector element. Light (ultraviolet, visible or infrared) can be transmitted through the glass plate into the sample, back-reflected from the principal detector element, transmitted back through the sample and analyzed for absorption or fluorescence.
In yet another embodiment of the present invention, a semi-permeable membrane can be analyzed by using a principal detector element that is permeable. The principal detector element is a generally circular, square, or oval-shaped, flat metal conductor with a plurality of holes extending through it that is secured to electrically conductive rods and disposed within the toroid cavity. In this embodiment, a semi-permeable membrane is positioned against one flat side of the principal detector element. A sample (gas, liquid, or solid) encircled by an O-ring is positioned between that semi-permeable membrane and a glass plate. Another semi-permeable membrane is positioned against the opposite flat side of the principal detector element. Another sample (gas, liquid, or solid) encircled by an O-ring is positioned between that other semi-permeable membrane and another glass plate. The glass plates are compressed towards each other by the tightening of nonconductive screws that extend through aligned holes in the glass plates into nonconductive nuts. As a result, the samples and O-rings on both sides of the principal detector element are compressed against the semi-permeable membrane on the respective sides of the principal detector element. An osmotic pressure can be developed across the semi-permeable membrane so that atoms, molecules, and/or ions pass through the semi-permeable membrane and reach the principal detector element. As a result, the material that has passed through the membrane is placed directly at the most sensitive region of the toroid cavity detector, i.e., in close proximity to the principal detector element. This material can be analyzed and identified according to its spatial location by an imaging procedure or by a measured signal enhancement.